Preserve function name case from SQL source in EXPLAIN AST output

ClickHouse EXPLAIN AST preserves the original case of function names as
written in the SQL source (e.g., CEIL stays CEIL, COALESCE stays COALESCE).
Only a few functions are normalized to specific canonical forms (e.g.,
DATEDIFF→dateDiff, POSITION→position, SUBSTRING→substring).

This fixes issues where special parser functions (parseIfFunction,
parseExtract, parseSubstring, parseArrayConstructor) were hardcoding
lowercase names instead of preserving the original case from the token.
Also fixes parseKeywordAsFunction which was incorrectly lowercasing all
keyword-based function names.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

internal/explain/format.go

@@ -448,24 +448,22 @@ func formatExprForType(expr ast.Expression) string {
 // NormalizeFunctionName normalizes function names to match ClickHouse's EXPLAIN AST output
 func NormalizeFunctionName(name string) string {
 	// ClickHouse normalizes certain function names in EXPLAIN AST
-	// Note: lcase, ucase, mid are preserved as-is by ClickHouse EXPLAIN AST
+	// Most functions preserve their original case from the SQL source.
+	// Only a few are normalized to specific canonical forms.
 	normalized := map[string]string{
-		"trim":     "trimBoth",
-		"ltrim":    "trimLeft",
-		"rtrim":    "trimRight",
-		"ceiling":  "ceil",
-		"log10":    "log10",
-		"log2":     "log2",
-		"rand":     "rand",
-		"ifnull":   "ifNull",
-		"nullif":   "nullIf",
-		"coalesce": "coalesce",
-		"greatest": "greatest",
-		"least":    "least",
+		// TRIM functions are normalized to trimBoth/trimLeft/trimRight
+		"trim":  "trimBoth",
+		"ltrim": "trimLeft",
+		"rtrim": "trimRight",
+		// CONCAT_WS is normalized to concat
 		"concat_ws": "concat",
+		// Position is normalized to lowercase
 		"position": "position",
-		"date_diff":  "dateDiff",
-		"datediff":   "dateDiff",
+		// SUBSTRING is normalized to lowercase (but SUBSTR preserves case)
+		"substring": "substring",
+		// DateDiff variants are normalized to camelCase
+		"date_diff": "dateDiff",
+		"datediff":  "dateDiff",
 		// SQL standard ANY/ALL subquery operators - simple cases
 		"anyequals":    "in",
 		"allnotequals": "notIn",

parser/expression.go

@@ -1596,7 +1596,8 @@ func (p *Parser) wrapWithAlias(expr ast.Expression, alias string) ast.Expression
 
 func (p *Parser) parseExtract() ast.Expression {
 	pos := p.current.Pos
-	p.nextToken() // skip EXTRACT
+	name := p.current.Value // preserve original case
+	p.nextToken()           // skip EXTRACT
 
 	if !p.expect(token.LPAREN) {
 		return nil
@@ -1646,7 +1647,7 @@ func (p *Parser) parseExtract() ast.Expression {
 			p.expect(token.RPAREN)
 			return &ast.FunctionCall{
 				Position:  pos,
-				Name:      "extract",
+				Name:      name,
 				Arguments: args,
 			}
 		}
@@ -1670,7 +1671,7 @@ func (p *Parser) parseExtract() ast.Expression {
 
 	return &ast.FunctionCall{
 		Position:  pos,
-		Name:      "extract",
+		Name:      name,
 		Arguments: args,
 	}
 }
@@ -1764,7 +1765,8 @@ func (p *Parser) parsePositionalParameter() ast.Expression {
 
 func (p *Parser) parseSubstring() ast.Expression {
 	pos := p.current.Pos
-	p.nextToken() // skip SUBSTRING
+	name := p.current.Value // preserve original case
+	p.nextToken()           // skip SUBSTRING
 
 	if !p.expect(token.LPAREN) {
 		return nil
@@ -1871,7 +1873,7 @@ func (p *Parser) parseSubstring() ast.Expression {
 
 	return &ast.FunctionCall{
 		Position:  pos,
-		Name:      "substring",
+		Name:      name,
 		Arguments: args,
 	}
 }
@@ -2705,7 +2707,8 @@ func (p *Parser) parseQualifiedColumnsMatcher(qualifier string, pos token.Positi
 
 func (p *Parser) parseArrayConstructor() ast.Expression {
 	pos := p.current.Pos
-	p.nextToken() // skip ARRAY
+	name := p.current.Value // preserve original case
+	p.nextToken()           // skip ARRAY
 
 	if !p.expect(token.LPAREN) {
 		return nil
@@ -2720,14 +2723,15 @@ func (p *Parser) parseArrayConstructor() ast.Expression {
 
 	return &ast.FunctionCall{
 		Position:  pos,
-		Name:      "array",
+		Name:      name,
 		Arguments: args,
 	}
 }
 
 func (p *Parser) parseIfFunction() ast.Expression {
 	pos := p.current.Pos
-	p.nextToken() // skip IF
+	name := p.current.Value // preserve original case
+	p.nextToken()           // skip IF
 
 	if !p.expect(token.LPAREN) {
 		return nil
@@ -2742,15 +2746,15 @@ func (p *Parser) parseIfFunction() ast.Expression {
 
 	return &ast.FunctionCall{
 		Position:  pos,
-		Name:      "if",
+		Name:      name,
 		Arguments: args,
 	}
 }
 
 func (p *Parser) parseKeywordAsFunction() ast.Expression {
 	pos := p.current.Pos
-	name := strings.ToLower(p.current.Value)
-	p.nextToken() // skip keyword
+	name := p.current.Value // preserve original case
+	p.nextToken()           // skip keyword
 
 	if !p.expect(token.LPAREN) {
 		return nil

parser/testdata/01705_normalize_case_insensitive_function_names/metadata.json

@@ -1 +1 @@
-{"explain_todo":{"stmt2":true}}
+{}

parser/testdata/01710_minmax_count_projection/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt33": true
-  }
-}
+{}

parser/testdata/02416_rocksdb_delete_update/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt21": true
-  }
-}
+{}

parser/testdata/02577_keepermap_delete_update/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt21": true
-  }
-}
+{}

parser/testdata/02707_keeper_map_delete_update_strict/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt22": true
-  }
-}
+{}

parser/testdata/03595_funcs_on_zero/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt58": true
-  }
-}
+{}